Three-phase power has been the primary method of power distribution for many years. However, for many uses, such as incandescent lights, most home appliances, and various types of other electrical and electronic equipment, which present a single-phase load to the power source, it is necessary to acquire single phase power from the three-phase power being supplied in order to operate.
As the typical single-phase load requires a single-phase power source with which to operate, customarily a single-phase power transformer is connected across one of the three outputs of the three-phase power source to convert three-phase power into single-phase power for the purpose of operating the single-phase load. However, this configuration often results in excessive current distortion in the three-phase power source.
Another method for accomplishing this objective is to use three separate conventional single-phase transformers that are connected to the three-phase source. This method allows for a more evenly balanced current load and is less likely to cause current distortion than the use of only one single-phase transformer.
Three-phase "wye" transformers are also being used as a means of distributing single-phase power from a three-phase source. This is accomplished by interposing a delta-to-wye three-phase transformer between the three-phase source and the single-phase load. This method also provides for a more evenly divided current load and is less likely to cause current distortion than simply connecting one single-phase power transformer across two of the three phases of the power source.
These methods, however, may result in major problems in power quality which will adversely affect sophisticated electronic equipment, particularly such equipment in current use which constitutes a non-linear type load. Switching power supplies often found in digital electronic loads and fluorescent lighting are two examples of this non-linear loading. Among the problems non-linear loading creates are: excessive reactive currents, excessive EMI/RFI emissions, poor power factor, harmonic distortion and nuisance grounding currents.
Interposing a single-phase transformer having a secondary winding with a center tap thereon which is used for grounding purposes only to provide a balanced single-phase voltage source (e.g. 120/60V), as is shown in U.S. Pat. No. 5,640,314, creates an unbalanced current load on the three-phase source and the use of three such transformers is very cumbersome and expensive.